The invention relates generally to coupling between a collimated, free-space optical signal and a single-mode optical fiber, and in particular to optimizing the alignment and focus of an optical signal into a fiber core based on the measure of transmitted power in the fiber.
Although there have been many examples of active optical elements used in prior art for beam steering, adaptive lenses, and wavefront correction, the present invention is unique in its implementation, control approach simplicity, ability to correct dominant, low-order forms of wavefront error, and robustness. Some of the relevant prior art is discussed here to establish the novelty and utility of the current invention.
U.S. Pat. No. 4,572,616 by Kowel (issued in 1986) teaches an adaptive liquid crystal lens that can be electronically corrected for both internal and external aberrations. Kowel teaches that the refractive index of the liquid crystal device can be controlled to bring entering light to a focus upon exiting the device. It is claimed that the device can be made to exhibit characteristics of both a positive lens and a negative lens. Details of the control approach are not presented, except that a “control means” is used to drive the active optical element, with control signals being developed by “a processor in response to a given program” and “using well known microprocessor control techniques”. To correct for aberrations, a feedback loop was incorporated to command the liquid crystal lens based on measurements from a photoelectric sensor positioned after the device at its image plane. In contrast, the present invention uses a more efficient measurement of optical power within the fiber to which it is coupled, an adaptive controller is used to maximize power transfer, and a model-based feedback loop is not required, which avoids controller stability issues.
U.S. Pat. No. 5,071,253 by Chase (issued in 1991) teaches a light beam position control system that utilizes Kerr cells and Lummer-Gehrcke plates to produce sets of optical phased linear arrays for beam positioning and beam monitoring. This device is for beam steering only, but illustrates a common shortcoming of prior art, that being an efficient feedback signal for controlling the active device. In a Kerr cell, the index of refraction of a material can be changed by applying an electric field. Chase teaches a light beam control system wherein the angular position of the “controlled” beam is monitored by measurement of the angular position of a second beam. The output of the active cells and plates is split into a primary and secondary beam, the secondary beam being interrogated and used to adjust the voltage applied to the active elements to result in precise control of the primary beam. Chase teaches that the controller includes an image analysis means, including a television camera and means for generating first and second control signals, or an optical interface maximum fringe counter and digital counting means for determining monitor (secondary) beam position. Obviously, analysis of the monitor (secondary) beam is complex and no account is given of how angular position of the primary beam can be inferred or is related to analysis of the secondary beam. In contrast, the present invention uses a simpler and more robust approach for adaptively optimizing coupling with the fiber. Optical phased arrays require lower voltage levels than Kerr cells and Lummer-Gehrcke plates, and can be used for focusing a beam.
U.S. Pat. No. 5,233,673 by Vali (issued in 1993) relates to an optical phased array used to steer and focus an optical beam. However, instead of using a linear or planar optical phased array device to focus light energy into a fiber-optic, Vali teaches the use of an array of fiber-optic cables, wherein the relative phases along the optical paths are adjusted so that the light emitted from the fiber-optics is effectively focused. In contrast, the present invention uses an optical phased array device to focus light into a fiber-optic cable.
U.S. Pat. No. 6,128,421 by Roberts relates to beam steering using electro-optic phase modulators, optical phase array emitters, and electronic control circuits. Roberts states that a key problem with prior art approaches to electric beam steering is that control systems have been prohibitively complex. In contrast, the present invention uses a measurement of the optical power transmitted into the fiber to adaptively optimize the focal length and steered angle of the optical phased array. This allows tremendous flexibility, and the controller is able to compensate for dominant sources of wavefront error expected to be encountered during its service life.
U.S. Pat. No. 6,597,836 by Johnson teaches a control system for optical phased arrays that compensates for phase changes due to external perturbations such as vibration and thermal variations, as well as compensating for atmospheric aberrations external to the laser system. Johnson teaches an approach for compensating for wavefront errors as opposed to beam steering and focusing. Johnson uses analysis of the wavefront exiting the active device (that device being an optical phased array or phase modulator) to generate a control signal that is fed back to the active device. Johnson teaches that algorithms which are used in the known art to calculate phase measurements and corrections are computationally intensive, and therefore may not be capable of providing real-time feedback to the phase modulators. Johnson teaches the use of a personal computer or a plurality of personal computers to conduct analysis of the exiting beam to generate the necessary control signal. In contrast, the present invention does not require a measurement of the transmitted wavefront and subsequent analysis, which is prohibitively complex and computationally intensive. It will be shown that by maximizing the optical power transmitted into the fiber-optic, one is inherently compensating for the dominant modes of wavefront error, those being spherical aberration, tip, and tilt errors. Further, we avoid control stability issues by not using a feedback controller. Instead, an adaptive controller is used to make adjustments to the optical phased array in a feed-forward control architecture. This precludes the need to have an accurate system model for controller synthesis and allows for time-varying system dynamics.
U.S. Pat. No. 7,079,203 by Huang teaches an electronically tunable micro-lens device that uses an inhomogeneous, concentrated, polymer network with nemantic liquid crystal. Huang asserts that micro-lenses are one of the most important components in the micro-optics field for optical fiber coupling. Huang teaches a gradient-index liquid crystal lens with a tunable focal length and low voltage requirements. The device is polarization independent, and capable of being used as either a positive or negative lens. Huang suggests implementation of the device in applications spanning optical communications, fiber-optic coupling, adaptive optics, wavefront error correction, and electronic beam steering, although no discussion of practical implementation or limitations is presented. U.S. Pat. No. 7,327,434 by Ren also teaches an electronically tunable lens device that uses uneven, homogenous liquid crystal droplets. Ren teaches a gradient-index liquid crystal lens with a tunable focal length, independent of polarization, and capable of being used as either a positive or negative lens. Such devices as taught by Huang and Ren are directly applicable to optical phased arrays that can be used for wavefront correction, beam steering, and adaptive lensing. These inventions are cited to demonstrate the progressing state-of-the-art in optical phased array devices. It is possible to achieve polarization-independent, variable focusing, and precision beam steering devices. There is clearly a need for an improved method of using such devices to optimize coupling of optical energy into an optical fiber, one that uses a simple and efficient adaptive control approach.